Nonwoven webs are used in a variety of products, including the manufacture of medical fabrics, wiping cloths, and disposable personal care products, such as diapers, sanitary napkins and the like.
There are a wide variety of techniques and polymers used to produce nonwoven products. For example, nonwoven webs may be produced by meltblowing techniques. In meltblowing, a thermoplastic resin is fed into an extruder, heated, and then fed to a special melt-blowing die typically comprising a plurality of linearally arranged, small diameter capillaries. The resin emerges from the die orifices as molten threads into a high velocity stream of gas, usually air. The air attenuates the polymer into a blast of fine fibers and/or filaments, which are collected on a moving screen placed in front of the blast. The fibers entangle to form a cohesive web.
Typically, polymer properties affect both processing conditions and properties of the nonwoven web produced. For example, low molecular weight polymers exhibit lower viscosities and higher melt flow rates. Thus, these polymers attenuate more readily, allowing a high throughput of polymer. However, the resultant web is weak because the low molecular weight polymer is inherently weak. That is, the polymer relaxation time is shorter than the attenuation time (time from capillary exit to solidification). In contrast, high molecular weight polymers typically exhibit higher strength but also have high viscosities and lower melt flow rates. Thus, these polymers flow less readily and although a stronger web can be produced, processability is impaired.
A polymer's ability to attenuate rapidly to form fine diameter fibers or filaments with minimal breakage is another important consideration in choosing a polymer. Typically, a good meltblowing resin is composed of polymer chains of about the same size or molecular weight (or with a narrow range, i.e., a narrow molecular weight distribution or "MWD"). A poor meltblowing polymer is thought to have a wide range of different sized polymer chains (or a broad MWD). Thus, prior procedures have attempted to provide polymers with a low molecular weight and a narrow molecular weight distribution.
Molecular weight and molecular weight distribution are difficult parameters to control in conventional polymerizations, such as propylene polymerizations using a Ziegler-type catalyst. Control of such parameters requires use of chain terminators or transfer agents, and the results obtained are strongly dependent upon the polymerization conditions. Thus, varying the polymerization process to produce a polymer having a desired average weight and molecular weight distribution can be difficult.
Given the difficulties of conventional polymerization techniques, typically to prepare a processable polymer, efforts have shifted from controlling polymerization parameters to altering the resultant polymer. For example, a reactor-prepared polymer having a high molecular weight can be treated to provide a polymer having a molecular weight and molecular weight distribution within a desired range. Typically, the reactor-produced polymer is degraded, i.e., subjected to a molecular scission step using thermal, radiation or chemical degradation techniques.
Prior procedures have emphasized the importance of uniform decomposition of the reactor-produced polymer to provide a processable product having a lower average molecular weight and a narrow molecular weight distribution. For example, in chemical degradation processes, a peroxide-based catalyst is often used to attack the polymer chain. A narrow molecular weight distribution is achieved by uniformly mixing the polymer and the catalyst so that upon initiation, the catalyst attacks and, through a free radical mechanism, randomly cleaves the molecules. Since the free radical initiator is well mixed into the polymer prior to activation, the uniformity of degradation is enhanced.
U.S. Pat. No. 4,451,589 discloses one such process for degrading polymers. In this process, the polymer is degraded stepwise by first forming pellets of the polymer and the prodegradant under condition initiating a portion of the prodegradant. In a subsequent step, the pellets are processed at which time the remainder of the product reacts. The prodegradant must be dispersed uniformly to produce a low viscosity polymer. U.S. Pat. No. 3,940,379 discloses another such process, wherein the degradation of propylene polymers is controlled using oxygen and a peroxide.
Such procedures can inherently narrow the molecular weight distribution and reduce the weight average molecular weight of the polymer, thereby providing a processable polymer. However, although these materials do provide improved processability by lowering molecular weight and narrowing the molecular weight distribution, processability is nevertheless improved at the cost of fiber and web strength as discussed previously.